1. Field of the Invention
The present invention relates to a printing system, which has a head for recording three or more different inks that are mixable to express hues in a predetermined range and records multi-tone images with the inks discharged from the head onto a printing object. The present invention also pertains to a method of recording images and an ink cartridge attachable to such a printing system.
2. Description of the Related Art
Color printers with a head for recording a plurality of color inks on a printing object are widely used as an output device of a computer, which prints multi-color, multi-tone images processed by the computer. The following techniques are known as the method of recording inks on a printing object: a thermal transfer system that melts inks on ink ribbons and transfers the molten inks to a sheet of paper; an ink jet system that sprays colored ink solutions onto a sheet of paper; and an electrophotographic system that uses laser or another light to create a latent image on a photoconductive intermediary and transfers color toners to render the latent image visible. All such available techniques reproduce colors in a predetermined hue range by mixing a plurality of color inks. Three color inks, cyan, magenta, and yellow (CMY) are generally used for full-color printing.
There are several techniques available for printing multi-tone, multi-color images with a plurality of color inks. One available technique, which is adopted in the conventional printers, expresses the tone of a printed image by the density of dots (the frequency of appearance of dots per unit area) while fixing the size of dots formed on a sheet of paper by a stream of ink droplets. Another available technique adjusts the diameter of dots formed on a sheet of paper, in order to vary the density of ink per unit area. The advanced fine working of the head for creating ink particles has improved the density of dots formable per predetermined length or the variable range of dot diameters.
The improvement in printers has, however, so far been limited to 300 dpi through 720 dpi in printing density or resolution and several tens microns in particle diameter. This is significantly lower than the level of expression or resolution of silver photography, which has reached several thousands dpi on the film. Dots are sparsely formed in an area of low image density, that is, in an area of low density of dots to be printed. This increases the degree of granularity and makes the dots undesirably conspicuous. In printers for spraying liquid ink onto a paper, the total amount of ink sprayed per unit area is restricted by the absorbable volume of ink by the paper (generally referred to as ink duty). The restriction of ink duty is an issue to be cleared in printers using a plurality of color inks for color printing. The issue of ink duty is actualized especially in case that higher-density ink and lower-density ink are provided for the respective colors and the lower-density ink is used for printing low-tone areas in order to decrease the degree of granularity. Expression of a specific tone with the lower-density ink results in increasing the total amount of ink sprayed per unit area.